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While the fundamental biophysics of C3 photosynthesis is highly conserved across plants, substantial leaf structural and enzymatic variation translates into variability in rates of carbon assimilation. Although this variation is well documented, it remains poorly understood how photosynthetic rates evolve, and whether macroevolutionary changes are related to the evolution of leaf morphology and biochemistry. A substantial challenge in large-scale comparative studies is disentangling evolutionary adaptation from environmental acclimation. We overcome this by using a ‘macroevolutionary common garden’ approach in which we measured metabolic traits (Jmax and Vcmax) from 111 phylogenetically diverse species in a shared environment. We find substantial phylogenetic signal in these traits at moderate phylogenetic timescales, but this signal dissipates quickly at deeper scales. Morphological traits exhibit phylogenetic signal over much deeper timescales, suggesting that these are less evolutionarily constrained than metabolic traits. Furthermore, while morphological and biochemical traits (LMA, Narea and Carea) are weakly predictive of Jmax and Vcmax, evolutionary changes in these traits are mostly decoupled from changes in metabolic traits. This lack of tight evolutionary coupling implies that it may be incorrect to use changes in these functional traits in response to global change to infer that photosynthetic strategy is also evolving. [ABSTRACT FROM AUTHOR]

Genome-scale scans have revealed highly heterogeneous levels of divergence between closely related taxa in many systems. Generally, a small number of regions show high differentiation, with the rest of the genome showing no or only low levels of divergence. These patterns have been interpreted as evidence for ongoing speciation-with-gene-flow, with introgression homogenizing the whole genome except loci involved in reproductive isolation. However, as the number of selected loci increases, the probability of introgression at unselected loci decreases unless there is a transmission ratio distortion causing an over-representation of specific combinations of alleles. Here we examine the transmission of three 'speciation islands' that contain fixed differences between the M and S forms of the mosquito, Anopheles gambiae. We made reciprocal crosses between M and S parents and genotyped over 2000 F2 individuals, developing a hierarchical likelihood model to identify specific genotypes that are under- or over-represented among the recombinant offspring. Though our overall results did not match the expected number of F2 genotypes, we found no biased co-transmission among M or S alleles in the three islands. Our likelihood model did identify transmission ratio distortion at two of the three islands, but this distortion was small (approx. 3%) and in opposite directions for the two islands. We discuss how our results impinge on hypotheses of current gene flow between M and S and ongoing speciation-with-gene-flow in this system. [ABSTRACT FROM AUTHOR]

Ecologically important traits do not evolve without limits. Instead, evolution is constrained by the set of available and viable phenotypes. In particular, natural selection may only favour a narrow range of adaptive optima constrained within selective regimes. Here, I integrate data with theory to test whether selection explains phenotypic constraint. A global database of 599 plant species from 94 families shows that stomatal ratio, a trait affecting photosynthesis and defence against pathogens, is highly constrained. Most plants have their stomata on the lower leaf surface (hypostomy), but species with half their stomata on each surface (amphistomy) form a distinct mode in the trait distribution. A model based on a trade-off between maximizing photosynthesis and a fitness cost of upper stomata predicts a limited number of adaptive solutions, leading to a multimodal trait distribution. Phylogenetic comparisons show that amphistomy is the most common among fast-growing species, supporting the view that CO2 diffusion is under strong selection. These results indicate that selective optima stay within a relatively stable set of selective regimes over macroevolutionary time. [ABSTRACT FROM AUTHOR]